Description (adapted from the application): This project is designed to examine the long-term illness course in CFS, and the specific aims include: 1) to establish a longitudinal data base which will document the developmental trajectories of CFS, i.e., the changes in symptoms and quality of life over time; 2) to identify the risk factors that contribute to illness exacerbation as well as the variables which lead to improvement (emphasis will be placed on examining the contributions of social and behavioral factors to the progression of the illness); and 3) to determine if quality of life in the long-term is influenced by specific patterns of CFS symptomatology independent of their interactions with other risk factors. CFS patients and healthy controls will be evaluated every seven months for about three and a half years. Analyses will be conducted by a variety of statistical modeling techniques including structural equation models, hierarchical linear models, mixed effects models, correspondence analysis and other general linear models. They anticipate that the proposed study will be the first systematic investigation into a number of critical issues related to illness progression and long-term outcome in CFS. In addition, it will determine what factors can predict outcome and thus may guide treatment.